1. Field of the Invention
This invention relates generally to marine seismic exploration, and more particularly to the suppression of pressure waves which propagate in undesirable directions.
2. Description of the Related Art
In marine seismic exploration, a seismic survey ship is equipped with an energy source and a receiver for taking seismic profiles of an underwater land configuration. The action of taking profiles is often referred to as "shooting," due to the fact that explosive devices where commonly used for many years as energy sources. Normal production shooting takes place while the survey ship is moving at a constant speed, and can produce over 1000 profiles per day.
The energy source is designed to produce compressional (pressure) or P-waves that propagate through the water and into the underwater land formation, so that reflected energy can be received and processed into an image which provides information about the structure of the subterranean formation. Presently, one of the most common energy sources is an air gun which discharges air under very high pressure into the water. The discharged air forms a bubble which oscillates at a frequency within the seismic range. Another energy source which is frequently used is a marine vibrator. Marine vibrators typically include a pneumatic or hydraulic actuator which causes an acoustic piston to vibrate at a selected frequency. The vibrations of the acoustic vibrator produce pressure differentials in the water which generate seismic pulses free from spurious bubbles. Environmentalists prefer marine vibrators over air guns and explosives because the latter are harmful to marine life.
The receivers in marine applications are typically referred to as hydrophones. The hydrophones convert pressure waves into electrical signals which are used for analog or digital processing. The most common type of hydrophone is of the piezoelectric type. The hydrophones are mounted on a long streamer which is towed behind the survey ship at a depth of about 30 feet. A problem arises in that the energy sources, which are typically operated at similar depth, generate pressure waves that propagate radially outward. Therefore, the hydrophones receive horizontally propagating energy directly from the energy sources as well as energy reflected from the underwater land formation. This horizontally propagated energy interferes with the reception and analysis of the reflected energy.
The energy that is propagated in a horizontal direction relative to the energy sources is picked up not only by the receivers associated with the exploration platform that is generating the energy, but also by the receivers associated with other marine seismic exploration crews in the area. Thus, crews often must plan marine seismic operations so as to minimize interference. This is of particular concern in tee Gulf of Mexico and the North Sea.
Much work has been done in the design of air gun source arrays. The extended array described by Huizer (U.S. Pat. No. 4,727,956) describes an airgun array which is wide. Through proper selection of spacing and gun size his array could be designed to provide attenuation of horizontally propagating compressional wave energy at the upper end of the seismic frequency band. Most other work with marine source arrays has been devoted to spectral shaping of the downgoing wavelet (Dragoset, U.S. Pat. No. 4,739,858, etc.).
Other effort has been expended to reduce the receiver sensitivity to horizontally propagating energy. Most streamers in use today have receiver groups comprised of hydrophones. The hydrophones in common use are piezoelectric devices responsive to fluid pressure changes. They are omnidirectional transducers. An array of hydrophone elements is most often formed within the streamer, the array being usually comprised of evenly spaced elements. Some rejection of a wavefront propagating in the direction of the hydrophone array length will be realized. No rejection of broadside noise is possible for this case. Another method for achieving some rejection of horizontally propagating energy is to employ particle motion detectors (geophones or accelerometers). A geophone is a device responsive to particle velocity. Common single element geophones are of a moving coil construction and have a low cross-axis sensitivity. Use of such devices in a streamer requires gimble mounting to keep the geophones vertically oriented within the streamer as it twists. The gimbled geophones are therefore only sensitive to vertical motion and would tend to be insensitive to horizontal propagating energy. Gimbled geophones have many disadvantages: among them being cost and reliability. Recent patents for self-orienting geophones/accelerometers include: U.S. Pat. No. 4,345,473 and U.S. Pat. No. 4,618,949.
Signal processing is employed to provide further enhancement of the seismic reflections. Velocity filtering, and common-depth-point stacking are steps in the processing sequence which can be used to provide some additional suppression of horizontally propagating energy. While they can provide additional improvement in the signal-to-noise ratio, even greater improvement could be achieved when used in combination with data acquisition methods which suppress energy coming from outside the plane of interest.
The present invention is directed to overcoming one or more of the problems as set forth above.